Shocked Ambient Gas Research Articles

The structure of nearly isothermal, adiabatic shock waves

ABSTRACT An explosively generated shock wave with time-dependent radius R(t) is characterized by a phase in which the shocked gas becomes radiative with an effective adiabatic index γ ≃ 1. Using the result that the post-shock gas is compressed into a shell of width ΔR/R ≃ δ, where δ = γ − 1, we show that a choice of self-similar variable that exploits this compressive behaviour in the limit that γ → 1 naturally leads to a series expansion of the post-shock fluid density, pressure, and velocity in the small quantity δ. We demonstrate that the leading-order (in δ) solutions, which are increasingly accurate as γ → 1, can be written in simple, closed forms when the fluid is still approximated to be in the energy-conserving regime (i.e. the Sedov–Taylor limit), and that the density declines exponentially rapidly with distance behind the shock. We also analyse the solutions for the bubble surrounding a stellar or galactic wind that interacts with its surroundings, and derive expressions for the location of the contact discontinuity that separates the shocked ambient gas from the shocked wind. We discuss the implications of our findings in the context of the dynamical stability of nearly isothermal shocks.

X-RAY OBSERVATION OF THE SHOCKED RED SUPERGIANT WIND OF CASSIOPEIA A

Cas A is a Galactic supernova remnant whose supernova explosion is observed to be of Type IIb from spectroscopy of its light echo. Having its SN type known, observational constraints on the mass-loss history of Cas A's progenitor can provide crucial information on the final fate of massive stars. In this paper, we study X-ray characteristics of the shocked ambient gas in Cas A using the 1 Ms observation carried out with the Chandra X-Ray Observatory and try to constrain the mass-loss history of the progenitor star. We identify thermal emission from the shocked ambient gas along the outer boundary of the remnant. Comparison of measured radial variations of spectroscopic parameters of the shocked ambient gas to the self-similar solutions of Chevalier show that Cas A is expanding into a circumstellar wind rather than into a uniform medium. We estimate a wind density nH ~ 0.9 $\pm$ 0.3 cm$^{-3}$ at the current outer radius of the remnant (~3 pc), which we interpret as a dense slow wind from a red supergiant (RSG) star. Our results suggest that the progenitor star of Cas A had an initial mass around 16 Msun, and its mass before the explosion was about 5 Msun, with uncertainties of several tens of percent. Furthermore, the results suggest that, among the mass lost from the progenitor star (~11 Msun), a significant amount (more than 6 Msun) could have been via its RSG wind.

Experimental investigation of the structure and the dynamics of nanosecond laser-induced plasma in 1-atm argon ambient gas

We have investigated the structure and the dynamics of the plasma induced on a metallic target in 1-atm argon ambient by a nanosecond laser pulse with irradiance in the range of 10 GW/cm2. The structure is revealed to be sensitively dependent on the laser wavelength. A layered structure of different species characterizes the plasma induced by ultraviolet 355 nm pulse, while an effective mixing between the ablation vapor and the shocked ambient gas is observed with infrared 1064 nm pulse. The absorption property of the shocked gas is found to be crucial for determining the structure of the plasma.

SNR 0104-72.3: A REMNANT OF A TYPE Ia SUPERNOVA IN A STAR-FORMING REGION?

We report our 110 ks Chandra observations of the supernova remnant (SNR) 0104-72.3 in the Small Magellanic Cloud (SMC). The X-ray morphology shows two prominent lobes along the northwest-southeast direction and a soft faint arc in the east. Previous low resolution X-ray images attributed the unresolved emission from the southeastern lobe to a Be/X-ray star. Our high resolution Chandra data clearly shows that this emission is diffuse, shock-heated plasma, with negligible X-ray emission from the Be star. The eastern arc is positionally coincident with a filament seen in optical and infrared observations. Its X-ray spectrum is well fit by plasma of normal SMC abundances, suggesting that it is from shocked ambient gas. The X-ray spectra of the lobes show overabundant Fe, which is interpreted as emission from the reverse-shocked Fe-rich ejecta. The overall spectral characteristics of the lobes and the arc are similar to those of Type Ia SNRs, and we propose that SNR 0104-72.3 is the first case for a robust candidate Type Ia SNR in the SMC. On the other hand, the remnant appears to be interacting with dense clouds toward the east and to be associated with a nearby star-forming region. These features are unusual for a standard Type Ia SNR. Our results suggest an intriguing possibility that the progenitor of SNR 0104-72.3 might have been a white dwarf of a relatively young population.

A molecular survey of outflow gas: velocity-dependent shock chemistry and the peculiar composition of the EHV gas

(Abridged) We present a molecular survey of the outflows powered by L1448-mm and IRAS 04166+2706, two sources with prominent wing and extremely high velocity (EHV) components in their CO spectra. The molecular composition of the two outflows presents systematic changes with velocity that we analyze by dividing the outflow in three chemical regimes, two of them associated with the wing component and the other the EHV gas. The analysis of the two wing regimes shows that species like H2CO and CH3OH favor the low-velocity gas, while SiO and HCN are more abundant in the fastest gas. We also find that the EHV regime is relatively rich in O-bearing species, as is not only detected in CO and SiO (already reported elsewhere), but also in SO, CH3OH, and H2CO (newly reported here), with a tentative detection in HCO+. At the same time, the EHV regime is relatively poor in C-bearing molecules like CS and HCN. We suggest that this difference in composition arises from a lower C/O ratio in the EHV gas. The different chemical compositions of the wing and EHV regimes suggest that these two outflow components have different physical origins. The wing component is better explained by shocked ambient gas, although none of the existing shock models explains all observed features. The peculiar composition of the EHV gas may reflect its origin as a dense wind from the protostar or its surrounding disk.

Relativistic Rayleigh–Taylor instability of a decelerating shell and its implications for gamma-ray bursts

Global linear stability analysis of a self-similar solution describing the interaction of a relativistic shell with an ambient medium is performed. The solution is shown to be unstable to convective Rayleigh–Taylor modes having angular scales smaller than the causality scale. Longer wavelength modes are stable and decay with time. For modes of sufficiently large spherical harmonic degree l the dimensionless growth rate scales as , where Γ is the Lorentz factor of the shell. The instability commences at the contact interface separating the shocked ejecta and shocked ambient gas and propagates to the shocks. The reverse shock front responds promptly to the instability and exhibits rapidly growing distortions at early times. Propagation to the forward shock is slower, and it is anticipated that the region near the contact will become fully turbulent before the instability is communicated to the forward shock. The non-universality of the Blandford–McKee blast wave solution suggests that turbulence generated by the instability in the shocked ambient medium may decay slowly with time and may be the origin of magnetic fields over a long portion of the blast wave evolution. It is also speculated that the instability may affect the emission from the shocked ejecta in the early post-prompt phase of gamma-ray bursts.

CONVECTIVE INSTABILITY OF A RELATIVISTIC EJECTA DECELERATED BY A SURROUNDING MEDIUM: AN ORIGIN OF MAGNETIC FIELDS IN GAMMA-RAY BURSTS?

Global linear stability analysis of a self-similar solution describing a relativistic shell decelerated by an ambient medium is performed. The system is shown to be subject to the convective Rayleigh-Taylor instability, with a rapid growth of eigenmodes having angular scale much smaller than the causality scale. The growth rate appears to be largest at the interface separating the shocked ejecta and shocked ambient gas. The disturbances produced at the contact interface propagate in the shocked media and cause nonlinear oscillations of the forward and reverse shock fronts. It is speculated that such oscillations may affect the emission from the shocked ejecta in the early afterglow phase of gamma-ray bursts, and may be the origin of the magnetic field in the shocked circum-burst medium.

Could the Compact Radio Sources in M82 be Cluster Wind–driven Bubbles?

The compact non-thermal sources in M82 and other starburst galaxies are generally thought to be supernova remnants (SNRs). We consider an alternative hypothesis that most are wind driven bubbles (WDBs) associated with very young super star clusters (SSCs). In this scenario, the synchrotron emitting particles are produced at the site of the shock transition between the cluster wind and the hot bubble gas. The particles radiate in the strong magnetic field produced in the expanding shell of shocked ambient interstellar gas. One of the motivations for this hypothesis is the lack of observed time variability in most of the sources, implying ages greater than expected for SNRs, but comfortably within the range for WDBs. In addition, as SNRs, these sources are not effective in driving the starburst mass outflow associated with the nuclear region of M82, thus requiring a separate mechanism for coupling SN energy to this outflow. The WDB hypothesis is found to be feasible for underlying clusters in the mass range ~2x10^(4+/-1)Msun, and ambient gas densities in the range ~3x10^(3+/-1)cm^-3. The ages of the bubbles are between several x10^3 and several x10^4 years. Since the SNR picture cannont be ruled out, we provide suggestions for specific observational tests which could confirm or rule out the WDB hypothesis. Finally, we discuss the WDB hypothesis in the context of broader phenomena in M82, such as the rate of star formation and starburst outflows, and the possible interpretation of supershells in M82 as the products of multiple supernovae in young SSCs.

Evolution of Global Properties of Powerful Radio Sources. II. Hydrodynamical Simulations in a Declining Density Atmosphere and Source Energetics

Evolution of Global Properties of Powerful Radio Sources. II. Hydrodynamical Simulations in a Declining Density Atmosphere and Source Energetics

An unusual Hα nebula around the nearby neutron star RX J1856.5-3754

We present spectroscopy and Hα imaging of a faint nebula surrounding the X-ray bright, nearby neutron star RX J1856.5-3754 . The nebula shows no strong lines other than the hydrogen Balmer lines and has a cometary-like morphology, with the apex being approximately 1 ahead of the neutron star, and the tail extending up to at least 25 behind it. We find that the current observations can be satisfactorily accounted for by two different models. In the first, the nebula is similar to Balmer-dominated cometary nebulae seen around several radio pulsars, and is due to a bow shock in the ambient gas arising from the supersonic motion of a neutron star with a relativistic wind. In this case, the emission arises from shocked ambient gas; we find that the observations require an ambient neutral hydrogen number density n_(H^0) ≃ 0.8 cm^(-3) and a rotational energy loss Ė ≃ 6 × 10^(31) erg s^(-1). In the second model, the nebula is an ionisation nebula, but of a type not observed before (though expected to exist), in which the ionisation and heating are very rapid compared to recombination and cooling. Because of the hard ionising photons, the plasma is heated up to ~70 000 K and the emission is dominated by collisional excitation. The cometary morphology arises naturally as a consequence of the lack of emission from the plasma near and behind the neutron star (which is ionised completely) and of thermal expansion. We confirm this using a detailed hydrodynamical simulation. We find that to reproduce the observations for this case, the neutral hydrogen number density should be n_(H^0) ≃ 3 cm^(-3) and the extreme ultraviolet flux of the neutron star should be slightly in excess, by a factor ~1.7, over what is expected from a black-body fit to the optical and X-ray fluxes of the source. For this case, the rotational energy loss is less than 2 × 10^(32) erg s^(-1). Independent of the model, we find that RX J1856.5-3754 is not kept hot by accretion. If it is young and cooling, the lack of pulsations at X-ray wavelengths is puzzling. Using phenomenological arguments, we suggest that RX J1856.5-3754 may have a relatively weak, few 10^(11) G, magnetic field. If so, it would be ironic that the two brightest nearby neutron stars, RX J1856.5-3754 and RX J0720.4-3125 , may well represent the extreme ends of the neutron star magnetic field distribution, one a weak field neutron star and another a magnetar.

Numerical simulations of magnetized jets

The present axisymmetric numerical simulations of light hypersonic jets allow unmagnetized jets and jets carrying a dynamically important magnetic field to be contrasted. After decelerating a weakly magnetized jet through a series of weak, oblique shocks, a Mach disk and a strong annular shock are encountered near the outer edges of the contact discontinuity separating the shocked fluid from the shocked ambient gas. Upon passing the annular shock, the gas quickly expands and enters a backflowing cocoon surrounding the jet. The overall speed of advance of the jet is reduced; matter near the jet axis which passes through the terminal Mach disk accumulates in a plug, and gas is discharged into the cocoon by the intermittent shedding of vortices. When magnetic stresses dominate, however, the jet is rapidly decelerated via a Mach disk and strong annular shock.

Self-consistent models for the X-ray emission from supernova remnants - an application to Kepler's remnant

A tool for exploring the evolution of X-ray emission from young SNRs is presented which employs a novel approach to the problem of time-dependent ionization in a shock-heated plasma. The solution of this problem is coupled to a spherically symmetric hydrodynamic calculation for the evolution of a point explosion in a uniform medium. The method is applied to Kepler's SNR, and two narrowly constrained classes of models which can simultaneously fit the spectral and morphological features of the object are found. One of these is a Sedov model in which the emission arises from shocked ambient gas, and the other is a reverse-shock model in which the SN ejecta is the dominant source of radiation. The emission from one specific model in each class is compared with the radial surface brightness profile, the 0.2-4.5 keV broadband spectrum, and the 1-3 keV moderate-resolution spectrum of the remnant. Reasonable fits are obtained in both cases.